It is well known that defective vision can be corrected by reshaping the cornea of the eye. Further, it is known that reshaping of the cornea can be accomplished in several ways. For example, the well known radial keratotomy procedure is used to establish weakened areas in the cornea which respond to internal pressure in the eye to move the cornea in its optical relationship with the retina. Another way in which vision can be corrected is by procedures which actually remove portions of the cornea to alter its optical properties. For the category of procedures wherein portions of the cornea are removed in order to directly alter its optical properties, there is an increased appreciation that lasers may be efficacious as a surgical tool. U.S. Pat. No. 4,732,148 and U.S. Pat. No. 4,773,414 which issued to L'Esperance Jr. for inventions entitled "Methods for Performing Ophthalmic Laser Surgery" and "Method of Laser-Sculpture of the Optically Used Portion of the Cornea", respectively, are both exemplary of efforts to use laser beams for ophthalmic surgery on the cornea. These procedures, however, require there be some initial mechanical removal of portions of the cornea as preparation for the subsequent removal of corneal tissue by photoablation. As will be readily appreciated by the skilled artisan, such a requirement necessitates the use of different surgical tools in the same operation. It is, of course, preferable if the same surgical tool can be used throughout the procedure. The present invention recognizes that a laser beam can be such a tool for the purpose of ophthalmic surgery which reshapes the cornea. In order to be effective as a surgical tool, however, laser beams must be precisely controlled. Thus, their operative characteristics must be carefully selected and these characteristics must be variable to meet the particular needs of the particular procedure.
In light of the above, it is an object of the present invention to provide a method for reshaping the cornea of the eye in which the pulse energy density or the wavelength of a pulsed laser beam can be varied to precisely control the photoablation of corneal tissue. Another object of the present invention is to provide a method for reshaping the cornea of the eye in which the removal of a precisely predetermined volume of corneal tissue is accomplished by a two-stage photoablation procedure that first takes away relatively large portions of corneal tissue and subsequently takes away relatively small portions of corneal tissue. Still another object of the present invention is to provide a method for reshaping the cornea of the eye using a pulsed laser beam in which the pulse energy density is relatively low. Yet another object of the present invention is to provide a method for reshaping the cornea of the eye which minimizes or avoids the adverse side effects caused when photoablation of corneal tissue is accomplished using a pulsed laser beam with pulse energy densities in the plasma regime. Another object of the present invention is to provide a method for reshaping the cornea of the eye which does not involve or require the mechanical removal of corneal tissue. Still another object of the present invention is to provide a method for reshaping the cornea of the eye which is an essentially continuous operation. Yet another object of the present invention is to provide a method for reshaping the cornea of the eye which is simple to accomplish and which is relatively cost effective.